1. Field
Example embodiments are directed to a non-volatile memory device using a variable resistance element.
2. Description of the Related Art
Non-volatile memory devices using a resistive material include, for example, phase change random access memories (PRAMs), resistive RAMs (RRAMs), magnetic RAMs (MRAMs) and the like. Dynamic RAMs (DRAMs) or flash memory devices use charge to store data, while the non-volatile memory devices using a resistive material store data using a variation in the phase or state of a phase change material, such as a chalcogenide alloy (PRAM), a variation in the resistance of a variable resistive element (RRAM), or a variation in the resistance of an MTJ (magnetic tunnel junction) film caused by the magnetized state of a ferromagnetic material (MRAM).
In the non-volatile memory device using a phase change material, the material changes from a crystalline state to an amorphous state or vice-versa depending on the change in temperature. The crystalline phase of the material has low resistance, while the amorphous phase has high resistance. The crystalline phase may define a first logic level, for example, set data or data of 0, and the amorphous phase may define a second logic level, for example, reset data or data of 1.
In addition, the phase change memory device applies a set pulse or a reset pulse to the phase change material to generate heat and perform a write operation. To write reset data, the phase change memory device applies the reset pulse to the phase change material to heat it to a temperature higher than its melting point, and rapidly cools the phase change material into an amorphous state. To write set data, the phase change memory device applies the set pulse to the phase change material to heat it to a temperature higher than the crystallization point, but below the melting point, maintains the temperature for a predetermined time, and then cools the phase change material transforming it into a crystalline state. In this case, the time required to write the set data is around five times longer than that required to write the reset data. For example, 600 ns may be required to write the set data, and 120 ns may be required to write the reset data.